The present invention relates to an optical coupling circuit, a light-receiving apparatus of the optical coupling circuit and a signal processing device
An optical coupling circuit (also referred as a photocoupler) is known as a device capable of transmitting a signal while electrically insulating between an input side and an output side. The optical coupling circuit is used in various systems such as an AC servo or an inverter.
The optical coupling circuit inputs an input electric signal to a light-emitting device and converts the input electric signal into an optical signal. The optical signal is received by a light-receiving device that is electrically insulated with the light-emitting device, and converted into an electric signal. The converted electric signal is output to the output side as an output signal.
Thus, it is desirable that the optical coupling circuit can accurately convert the input electric signal into the output signal. However, in general, a capacitive coupling occurs between a light-emitting unit and a light-receiving unit in the optical coupling circuit. Therefore, a noise propagates to the light-receiving unit from the light-emitting unit due to the capacitive coupling. For example, when the noise is large and a signal caused by the noise surpasses a threshold that is a reference for generating the output signal in the light-receiving unit, the case where the output signal is faultily output occurs. Thus, it is required that an effect of the noise is suppressed in the optical coupling circuit. For example, characteristics referred to as Common Mode Rejection (CMR) is required as an index indicating an extent of malfunction prevention with respect to the noise signal propagating between the light-emitting unit and light-receiving unit in the optical coupling circuit.
An optical signal receiving circuit, which generates a voltage signal corresponding to the noise by adding a dummy photo diode and a transimpedance amplifier to eliminate the voltage signal corresponding to the noise from an output signal, is provided as an optical coupling circuit that has noise immunity (Japanese Unexamined Patent Application Publication No. 2006-340072).
In this example, a first transimpedance amplifier converts a first current signal generated by a first photo diode to which an optical signal is input into a first voltage signal. A second transimpedance amplifier converts a second current signal generated by a second photo diode that is configured so that the optical signal is not input into a second voltage signal. The first voltage signal and second voltage signal are input to a comparator and a comparison result is output as the output signal.
In this optical signal receiving circuit, when a noise signal is detected, a voltage of one of the first voltage signal and second voltage signal is shifted to increase a voltage difference between thereof. Thus, the effect of the noise is canceled by the comparator. As a result, the effect of the noise can be eliminated from the output signal.